This invention relates to an apparatus and method therefor, which includes the deep boring into a formation for the purpose of extracting hydrocarbon fluids from the earth. More specifically, the invention relates to a method for thermally treating a subterranean formation to reduce the viscosity of fluids which are retained in the formation, thereby enhancing the extraction of the hydrocarbons. The latter is achieved through the use of means to control the movement of the thermal treating reaction, to most effectively sweep a productive layer.
The vast majority of heavy oil reservoirs such as tar sand formation, lie 50 meters under the surface of the earth and therefore must be tapped through the use of in situ technology. Examples of heavy oil reservoirs include those located at the Athabasca and Cold Lake regions of Canada. In the United States the Edna and Sisquoqe are located in California as well as in the tar sand triangle in Utah.
The high viscosity of hydrocarbon having a density which is between 10 and 20 API, characterize all the above mentioned deposits and substantially all tar sand reservoirs. The heavy oil found in such reservoirs commonly referred to as bitumen, contained in tar sands, is usually immobile at reservoir temperatures.
In the particular situation of tar sand reservoirs such as in the Athabasca region of Canada, the reservoir temperature is approximately 7.degree. C. The viscosity of the viscous oil is above 1 million centipoises. In such a condition for all practical purposes, the hydrocarbon is considered as being formed of solid matter.
Another important feature of many tar sand reservoirs is the nature of the solid matrix which is generally comprised of a fine, unconsolidated sand with the median between 100 and 200 um containing the heavy, viscous oil.
Attempts to recover heavy oil from a tar sand formation have to deal with at least two major problems. These include the reduction of the oil's viscosity, and the movement of solids such as unconsolidated sands.
One of the most successfully applied practices for recovery of the heavy oil from tar sand is through in situ heating or stimulation.
The efficiency of the heating process is enhanced by the increase of net convection, or the amount of heating fluid which can penetrate the tar sand formation. As a rule, the tar sand's formation is initially saturated with an 80 to 90% high viscosity oil; only 10 to 20% saturated with gases and water.
For most of the tar sand reservoirs and particularly for ones with low and medium temperatures, an initial net convection of the heating fluids into the reservoir cannot be readily determined. Any increasing of the injection pressure of the heating or stimulating fluid carries with it the likelihood of fracturing the reservoir in an undesirable direction out of the tar sand formation. This circumstance is undesirable since the stimulating fluid will be of no further use.
To control the injection of heating fluids into the formation, it has been proposed to drill inclined and/or horizontally deviated wells. The latter will conduct the heating fluids, and/or additives directly into the heavy oil formation.
The present invention provides an improvement in means for producing bitumen and viscous hydrocarbon from tar sand and other formations in which the hydrocarbon is released through thermal stimulation. Examples of such wells are disclosed in U.S. Pat. No. 3,913,672, J. C. Allen et al; U.S. Pat. No. 3,908,762, D. A. Redford. In the methods there taught, naturally occurring high permeability breaks or those formed by means of conventional hydraulic fracturing and propping are proposed as a solution for penetrating the tar sand formation.
A diversified, simultaneous injection-production method can be achieved from a single well as disclosed in U.S. Pat. No. 3,813,671, D. A. Redford et al. This latter concept is further disclosed in U.S. Pat No. 4,088,188, R. H. Widmyer.
In the above described methods, at least one production well is completed to provide a separate path from the surface in order that a treating fluid can be introduced into a portion of an underlying hydrocarbon productive formation. The aqueous heating fluid can be injected into a portion of the formation adjacent to the production well on a timed progress basis. Therefore, continuous injection of the aqueous heating fluid into the formation can also be utilized.
Toward the further production of heavy hydrocarbons from environments such as tar sands, there are known processes which utilize horizontal or long deviated wells to recover heavy oil from tar sands. These however are subject to at least two major drawbacks. The latter include the control of solids, and the open line bypass affect.
A significant problem is usually encountered when dealing with the production of heavy oil from an unconsolidated formation when a long, horizontal or deviated slotted liner is utilized. Briefly, when the liner penetrates clean sand or shale zones, fine sand particles and bitumen which are displaced during the recovery process form a series of flow paths.
A further problem encountered in production from a horizontal well relates to the control of the recovery process when a long inclined or horizontal liner is introduced into the tar sand formation. Injected and produced fluids move in and out of the formation radially under the effects of a huff and puff external action and/or under the effect of condensation and flashing during thermal pumping of steam, a method referred to generally as steam stimulation.
Hot water, liquid additives and multiphase fluids that stay far from saturation, are not moved rapidly as easily as steam condensate which is near saturation. The only way to assure optimum penetration of stimulating liquids and/or gases in mixture, is by backing their injection with pressure. This mechanism is, however, less effective than flashing conduction thermal pumping.
In any tar sand substrate, the desired sweeping effect between the injection and production points along a horizontal well is reduced at a negligible fraction since the permeability to flow of an open line is usually many times higher than the permeability of the formation. Therefore, when hot fluids other than steam are injected, the formation around the previously stimulated liner in a cyclic steam injection, is cooled down. This results since only a very small fraction of fluids will be able to penetrate the formation.
The delicate balance between flow of hot stimulating fluid, and heat transfer, will eventually further reduce the actual flow rate into the formation to a very small insignificant area around the slotted perforated liner.
It is found therefore that if the horizontal well is able to radially distribute the injected steam during the stimulation step a reasonable, positive sweeping process can be maintained along the well. The long horizontal liner will tend to allow the hot fluids to bypass, thereby overriding the formation during axial flow between the injection and producing points along the liner.
There exists therefore an unfilled need for a system which will make a long horizontal or deviated well, into a more effectual system. Further, the well should be capable of better controlling both axial, and radial inflow-outflow of fluids.
In one embodiment of the presently disclosed invention, a highly deviated, or substantially horizontal wellbore is drilled to penetrate a tar sand formation. The wellbore can extend substantially through the center of the formation or along another appropriate level. A network of similar deviated wells of identical construction can be similarly drilled into the formation.
The particular spacing of wells and their arrangement will depend to a large degree on the size and the characteristics of the particular formation. One embodiment of the invention provides more than one well which extends through a substrate and which will penetrate the formation at an inclined alignment. Thereafter it can extend in a substantially horizontal disposition through the hydrocarbon productive layer.
To overcome the aforementioned problems in producing viscous hydrocarbons from a formation and in particular from a tar sand environment, there is presently provided an elongated, perforated well liner which is registered in a wellbore disposed within the subterranean hydrocarbon producing layer. A well head at one end of the liner permits pressured introduction of a stimulating fluid to the substrate.
An elongated fluid conduit is positioned in the liner, extending longitudinally thereof and having a discharge opening adjacent to the liner remote end. A diverter bed of randomly disposed metallic fibers surrounds a portion of said conduit to form a variable length, quasi-penetrable barrier in the liner. The diverter thereby segregates or separates the liner into injection and production segments or compartments. The barrier bed is progressively lengthened as the process proceeds whereby to assure a thorough thermal sweep of the formation along the well liner.
It is an object of the invention therefore to provide an improved means and method for producing hydrocarbon fluids from a substrate that requires thermal stimulation. A further object is to provide a well completion which is capable of providing an efficient sweep of the surrounding substrate by controlled introduction of stimulating fluid to the latter. A still further object is to provide a well completion in which a variable length diverter or quasi-barrier is utilized to most effectively distribute a hot stimulating fluid into the substrate whereby to provide a more efficient outflow of a hydrocarbon aqueous emulsion.